Patent Document 1 discloses a screw pump that has a pair of screw rotors engaged with each other. As the screw rotors rotate, the screw pump operates to transport fluid.
As shown in FIG. 11, a cross section of the tooth profile of a first conventional screw rotor 90A perpendicular to the rotor axis is shaped and sized equally with that of a second conventional screw rotor 90B. The cross section of the tooth profile of the first conventional screw rotor 90A perpendicular to the rotor axis is to the shape of the tooth profile of the first conventional screw rotor 90A on an imaginary plane extending perpendicular to the rotary axis of the first conventional screw rotor 90A. The cross section of the tooth profile of the first conventional screw rotor 90A perpendicular to the rotor axis includes a tooth top arc Q1R1, a tooth bottom arc S1T1, a first curve S1Q1, and a second curve T1R1. The first curve S1Q1 connects a first end S1 of the tooth bottom arc S1T1 to a first end Q1 of the tooth top arc Q1R1. The second curve T1R1 connects a second end T1 of the tooth bottom arc S1T1 to a second end R1 of the tooth top arc Q1R1.
The cross section of the tooth profile of the second conventional screw rotor 90B perpendicular to the rotor axis includes a tooth top arc Q2R2, a tooth bottom arc S2T2, a first curve S2Q2, and a second curve T2R2. The first curve S2Q2 connects a first end S2 of the tooth bottom arc S2T2 to a first end Q2 of the tooth top arc Q2R2. The second curve T2R2 connects a second end T2 of the tooth bottom arc S2T2 to a second end R2 of the tooth top arc Q2R2.
The first curve S1Q1 of the first conventional screw rotor 90A includes a trochoidal curve U1S1 and a connecting portion Q1U1. The trochoidal curve U1S1 is created by the path of the first end Q2 of the tooth top arc Q2R2 when the second conventional screw rotor 90B revolves about the first conventional screw rotor 90A. The connecting portion Q1U1 is a straight line that connects an end U1 of the trochoidal curve U1S1 to the first end Q1 of the tooth top arc Q1R1. The second curve T1R1 includes an outer circular arc R1W1, an involute curve W1Y1, and an inner circular arc Y1T1. The involute curve W1Y1 is located between the outer circular arc R1W1 and the inner circular arc Y1T1. The outer circular arc R1W1 is connected to the tooth top arc Q1R1 and the inner circular arc Y1T1 is connected to the tooth bottom arc S1T1.
Similarly, the first curve S2Q2 of the second conventional screw rotor 90B includes a trochoidal curve U2S2 and a connecting portion Q2U2, which is a straight line. The second curve T2R2 includes an outer circular arc R2W2, an involute curve W2Y2, and an inner circular arc Y2T2.
Neither the first conventional screw rotor 90A nor the second conventional screw rotor 90B contacts the housing of the screw pump. Further, the first conventional screw rotor 90A and the second conventional screw rotor 90B do not contact each other. Such arrangement thus may potentially cause leakage of the fluid (leakage of gas). Although the tooth profiles of the first and second conventional screw rotors 90A, 90B are shaped in such a manner as to suppress the fluid leakage, the fluid leakage is desired to be suppressed further effectively.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-351238